Sometimes, due to a PHP or other error, a Joomla component such as VirtueMart can stop working and just show a blank page / white screen (a WSOD). This lack of display hides the relevant error message.
When this happens check the server error logs (not access logs) if you have access to them. The error message will show what the problem was. The log files are often below website root in a dfirectory "logs"(!) and accessible via ftp or through the server cPanel. In many cases the location is defined in php.ini The following tutorial may help for VPS (and other server setups): http://www.123-reg.co.uk/support/answers/VPS/where-can-i-find-the-error-logs-on-my-vps-4068/
Or see this tutorial: http://www.ostraining.com/blog/joomla/white-screen-of-death
A WSOD can also be caused by a lack of PHP resources. Joomla + VM2/3 requires at least 128Mb of PHP memory, and preferably 256Mb.
You can also try this: Enable the vmdebug. It suppresses the fatal errors and allows debugging in many cases.